Metal fibers have a wide range of industrial applications. Specifically, metal fibers which retain their properties at high temperature and in corrosive environments may have application in capacitors, filtration media, and catalyst supports structures.
There has been increasing demand for miniature capacitors for the modern electronics industry. Capacitors comprising tantalum have been produced in small sizes and are capable of maintaining their capacitance at high temperatures and in corrosive environments. In fact, presently, the largest commercial use of tantalum is in electrolytic capacitors. Tantalum powder metal anodes are used in both solid and wet electrolytic capacitors and tantalum foil may be used to produce foil capacitors.
Tantalum may be prepared for use in capacitors by pressing a tantalum powder into a compact and subsequently sintering the compact to form a porous, high surface area pellet. The pellet may then be anodized in an electrolyte to form the continuous dielectric oxide film on the surface of the tantalum. The pores may be filled with an electrolyte and lead wires attached to form the capacitor.
Tantalum powders for use in capacitors have been produced by a variety of methods. In one method, the tantalum powder is produced from a sodium reduction process of K2TaF2. The tantalum product of sodium reduction can then be further purified through a melting process. The tantalum powder produced by this method may be subsequently pressed and sintered into bar form or sold directly as capacitor grade tantalum powder. By varying the process parameters of the sodium reduction process such as time, temperature, sodium feed rate, and diluent, powders of different particle sizes may be manufactured. A wide range of sodium reduced tantalum powders are currently available that comprise unit capacitances of from 5000 μF·V/g to greater than 25,000 μF·V/g.
Additionally, tantalum powders have been produced by hydrided, crushed and degassed electron beam melted ingot. Electron beam melted tantalum powders have higher purity and have better dielectric properties than sodium reduced powders, but the unit capacitance of capacitors produced with these powders is typically lower.
Fine tantalum filaments have also been prepared by a process of combining a valve metal with a second ductile metal to form a billet. The billet is worked by conventional means such as extrusion or drawing. The working reduces the filament diameter to the range of 0.2 to 0.5 microns in diameter. The ductile metal is subsequently removed by leaching of mineral acids, leaving the valve metal filaments intact. This process is more expensive than the other methods of producing tantalum powders and therefore has not been used to a wide extent commercially.
Additionally, the process described above has been modified to include an additional step of surrounding a billet substantially similar to the billet described above with one or more layers of metal that will form a continuous metal sheath. The metal sheath is separated from the filament array by the ductile metal. The billet is then reduced in size by conventional means, preferably by hot extrusion or wire drawing to the point where the filaments are of a diameter less than 5 microns and the thickness of the sheath is 100 microns or less. This composite is then cut into lengths appropriate for capacitor fabrication. The secondary, ductile metal that served to separate the valve metal components is then removed from the sections by leaching in mineral acids.
Further processing may be used to increase the capacitance of tantalum by ball milling the tantalum powders. The ball milling may convert substantially spherical particles into flakes. The benefit of the flakes is attributed to their higher surface area to volume ratio than the original tantalum powders. The high surface area to volume ratio results in a greater volumetric efficiency for anodes prepared by flakes. Modification of tantalum powders by ball milling and other mechanical processes has practical drawbacks, including increased manufacturing costs, and decrease in finished product yields.
Niobium powders may also find use in miniature capacitors. Niobium powders may be produced from an ingot by hydriding, crushing and subsequent dehydriding. The particle structure of the dehydrided niobium powder is analogous to that of tantalum powder.
Tantalum and niobium are ductile in a pure state and have high interstitial solubility for carbon, nitrogen, oxygen, and hydrogen. Tantalum and niobium may dissolve sufficient amounts of oxygen at elevated temperatures to destroy ductility at normal operating temperatures. For certain applications, dissolved oxygen is undesirable. Therefore, elevated temperature fabrication of these metal fibers is typically avoided.
Thus, there exists a need for an economical method for producing metal fibers. More particularly, there exists a need for an economical method for producing metal fibers comprising tantalum or niobium for use in capacitors, filter medium and catalyst supports, as well as other applications.